Keeper of the Matrix
From Tardis Wiki, the free Doctor Who reference
The Keeper of the Matrix was the Time Lord who watched over the Matrix and was in full charge of it. The Keeper kept the Key of Rassilon, the only key to the Matrix, on his person at all time. The Keeper was to keep the Matrix in good repair and make any necessary adjustments.
At the conclusion of the Sixth Doctor's trial, the Keeper had been taken over by the Valeyard. Possibly the real Keeper never left Gallifrey. (DW: The Ultimate Foe)
Some time later, Coordinator Engin became Keeper. (EDA: The Eight Doctors)
